During the production of hydrocarbons from underground rock formations, a tubing string extending from surface to the required depth in a well bore may be used to complete a well. To allow for different production operations or the downhole control of fluid flow, wireline tools such as plugs, chokes, safety valves, check valves, etc. may be placed in landing nipples in the tubing string. It is fairly common for such wireline tools to become stuck in the landing nipple. The general method used in attempting to retrieve such tools, or other objects stuck in a well bore, generally referred to as "fish", is through the use of a wireline pulling tool which is adapted to be attached to the wireline, and to engage the fish. In that method retrieval force is applied directly through the wireline. However, the strength of the wireline limits the amount of force available to dislodge the fish. If excessive force is applied, the wireline may break. In that case the wireline will fall to the bottom of the well and a very expensive fishing job is necessary to retrieve it.
Another method used to retrieve fish is the use of a "sucker rod" string which is run down the well via a service rig. Use of a sucker rod allows more force to be transmitted downhole. However, this method requires a very expensive operation.
One prior tool which is adapted to be utilized in conjunction with an operating string of tubing, is disclosed in U.S. Pat. No. 2,377,249 (Lawrence). That tool utilizes a series of cylinders with anchoring means for anchoring the cylinders to the well casing and with pistons in the cylinders connected to a grapple adapted to engage stuck pipe or other objects in a well. The pistons are designed to be actuated by liquid under pressure in the cylinders to exert a pull on the grapple and on the stuck object to dislodge the latter. The driving force for exerting the pull and anchoring the cylinder to the well casing is provided through operating fluid forced under pressure through the operating string from surface. While the tool is adapted to exert a force on the stuck object without exerting a force on the operating string, it is not adapted for use with wireline and involves the expense and difficulties encountered in running an operating string down a producing well.
A different system has been developed by Petro-Tech Tools Inc. in the United States. In that system, an explosive charge is detonated downhole in order to generate a dislodging force without all of the force being transmitted through the wireline. However, the explosive force cannot be adequately controlled, and damage may be caused to the tubing string, or the pulling tool.